Methods to produce high quality graphene (electronic/optoelectronic grade) include the silicon carbide (SiC) sublimation and the chemical vapour deposition (CVD). CVD is the manufacturing method with the greatest potential for future high quality large area graphene production. Graphene manufactured by CVD is expected to have a broad range of applications starting from electronics, optoelectronics, photonics, touch screen and display technology, and up to lighting to mention just a few.
Graphene can be manufactured using CVD methods on top of metal catalysts such as copper or nickel. The synthesis of monolayer graphene can be much better controlled using copper as the catalyst as opposed to nickel. Metal catalysts can be used in the form of foils or thin films deposited over other substrates. Graphene is typically formed within the CVD reactor at relatively high temperatures between 600° C. and up to 1000° C. and using gaseous carbon sources such as methane, ethylene or acetylene. Solid and liquid carbon sources can also be utilised, however the delivery of gases is more convenient and easier to control.
In order to make graphene suitable to be integrated in industrial processes it has to be transferred onto arbitrary substrates. This so-called transfer method involves the removal/detachment of the metal catalyst followed by placing the graphene on the final substrate. It is extremely important that the graphene film is intact, not damaged and retains all the required properties after the transfer process. Consequently, graphene should not be folded, broken or detached from the substrate and should be well positioned on it.
US20120244358A1 discloses a dry transfer method where the graphene film is directly transferred onto arbitrary substrates using direct printing methods. However, chemical modification of the receiving substrate is required and this could modify the properties of the graphene material. A homogeneous chemical modification of the substrate could be quite difficult to obtain and this could have a negative impact on the final graphene properties. In addition, this method is not suitable for large area monolayer graphene film (76 mm and above) transfer. The complete transfer printing to obtain close to 100% monolayer graphene on the final substrate would be extremely difficult.
US20110070146A1 discloses a manual wet transfer method where the graphene film along with the metal catalyst is separated from a silicon dioxide/silicon (SiO2/Si) substrate by applying a predetermined force or radiating ultrasonic waves in water. These wet transfer methods could damage the graphene layer since it is atomically thin.
US20120258311A1 discloses a roll based transfer method. This roll based method is adequate for flexible substrates; however that transfer method is not suitable for rigid substrates.
US20110200787A1 discloses another wet transfer method suitable for transferring very small graphene samples onto transmission electron microscopy (TEM) grids of 3 mm in diameter. The graphene is not protected with a sacrificial polymer layer and it is attached onto the TEM grid by the addition of a few drops of solvent. The graphene can easily break and get irreversibly damaged using this method. Consequently, this method is limited to very small samples (<3 mm discs).
In the graphene transfer methods of the prior art, graphene is manipulated manually. In addition, said methods are also limited to relatively small graphene area samples. Furthermore, standard laboratory glassware containers are used for the wet transfer of the graphene samples. The manual manipulation of graphene is extremely difficult and leads to low production yields due to operator related errors. The manipulation of a one atom thick film is not a trivial matter at all.
The technical problem to be solved can be seen in the provision of an automated graphene transfer method that includes tailor made equipment developed for the graphene transfer process and described herein, that increases production yield, reliability and reproducibility with respect to manual graphene transfer methods of the prior art.
A solution to this problem is provided by the method and equipment as defined in the claims of the present application.